Polyhydroxycarboxylic acids such as polylactic acid show excellent properties of heat resistance, molten moldability, toughness and rigidity. Of such materials, polylactic acid is synthesized from natural raw materials such as corn, and because it shows superior levels of transparency and biodegradability, it is attracting considerable attention as an environmentally friendly resin, and particularly as a resin for molding purposes. However, because it shows poor impact resistance and flexibility, and is brittle, industrial applications thereof are limited.
Techniques aimed at improving these shortcomings include techniques involving forming blends of polylactic acid with other resins or the like, or forming copolymers from polylactic acid and other resins or the like, and techniques in which a plasticizer is added to the polylactic acid.
One example of a process for forming blends of polylactic acid and other resins or the like is a process in which polylactic acid an aliphatic polyester produced from a diol and a dicarboxylic acid and a polycaprolactone, are subjected to molten blending (for example, see non-patent reference 1). However, in this process, in order to achieve satisfactory levels of properties such as impact resistance or flexibility, the quantities added of the other resins must be considerably large. As a result, the inherent transparency and heat resistance of the polylactic acid can't be kept, and because the compatibility with the polylactic acid is poor, problems such as bleeding can arise.
Furthermore, one example of a known process for producing a copolymer of polylactic acid and another resin or the like is a production process in which a lactide, which is the cyclic dimer of lactic acid, and an aliphatic polyester produced from a diol and a dicarboxylic acid are subjected to a ring-opening copolymerization in the presence of a catalyst (for example, see patent reference 1). However, the thus obtained copolymer is not entirely satisfactory and suffers from bleeding of the aliphatic polyester component.
On the basis of further investigations into the above processes for forming copolymers from polylactic acid and other resins or the like, the inventors of the present invention discovered that a lactic acid based copolymer produced by copolymerizing an aliphatic polyester, obtained by reacting a diol of 20 to 45 carbon atoms, and a dicarboxylic acid of 20 to 45 carbon atoms and/or 1,4-cyclohexanedicarboxylic acid, with a lactide in the presence of a ring-opening polymerization catalyst showed excellent transparency and good resistance to bleeding, and was useful as a packaging material or a molding material (for example, see patent reference 2). In addition, they also discovered that adding the above copolymer to polylactic acid yielded an impact resistance imparting agent that improved the impact resistance of the polylactic acid (for example, see patent reference 3), although in both these cases, depending on the intended application, satisfactory levels of impact resistance and flexibility were not always able to be obtained.
On the other hand, production processes have also been disclosed which do not use cyclic dimers such as lactides, but rather subject a polyhydroxycarboxylic acid and an aliphatic polyester to direct or indirect copolymerization (for example, see patent reference 4). In this production process, a polyhydroxycarboxylic acid, a polyester, a molecular weight increasing agent, and where necessary a chelating agent and/or an acidic phosphate ester are all combined, and by subjecting the mixture to molten mixing under reduced pressure while residual volatile matter is removed, a polyhydroxycarboxylic acid copolymer composition of increased molecular weight can be produced within a short time period.
More specific production processes include a process in which a polyhydroxycarboxylic acid such as polylactic acid, a polyester obtained by reacting a dicarboxylic acid and a diol, and a molecular weight increasing agent are mixed together to effect an increase in molecular weight, and a process in which a molecular weight increasing agent and a polyester obtained by reacting a dicarboxylic acid and a diol are first mixed together in advance to effect an increase in molecular weight, and a polyhydroxycarboxylic acid is then mixed in afterwards, either in the presence of, or in the absence of, a transesterification catalyst.
In the case of the former process, although the product depends on the type of molecular weight increasing agent used, and the point at which the chelating agent and/or acidic phosphate ester is added, if the chelating agent and/or acidic phosphate ester is added at the same time as the polyhydroxycarboxylic acid, the polyester, and the molecular weight increasing agent, then for example in the case where an organometallic material such as a tin or titanium based organometallic material is used as the molecular weight increasing agent, the organometallic material and the chelating agent and/or acidic phosphate ester undergo chelation, causing a deactivation of the organometallic material and a loss in the effect achieved by adding the molecular weight increasing agent.
Furthermore, if the polyhydroxycarboxylic acid, the polyester, and the molecular weight increasing agent are first mixed together, and the chelating agent and/or acidic phosphate ester is added afterwards, then when the polyhydroxycarboxylic acid and the polyester formed from the dicarboxylic acid and the diol are melted, the action of the residual esterification catalyst left over from the production of the polyester causes a depolymerization of the polyhydroxycarbokylic acid, causing problems such as coloring of the product polyhydroxycarboxylic acid copolymer composition, and difficulty in achieving a stable molecular weight. For example, in those cases where an organometallic material, and particularly a tin based organometallic material, is used as the molecular weight increasing agent, the depolymerization of the polyhydroxycarboxylic acid appears to be further accelerated.
Similarly, in the case of the latter of the production processes described above, the action of residual esterification catalyst, left over from the production of the polyester by the reaction between the dicarboxylic acid and the diol, causes problems such as depolymerization of the polyhydroxycarboxylic acid, and an increased likelihood of gelling.
Furthermore, other problems include an inability to produce a polyhydroxycarboxylic acid copolymer composition within a short time period, instability in the quality of the product due to a difficulty in determining the end point of the reaction, and the presence of unreacted raw material in the product, which can bleeding during production of sheets or films for example, meaning the polyhydroxycarboxylic acid copolymer compositions produced using the production processes described above still require considerable improvement.
As described above, conventional techniques have been unable to impart a practical level of impact resistance and flexibility to a polyhydroxycarboxylic acid while retaining a superior level of transparency.
(Non-patent Reference 1)
A. J. Domb, J. Polym. Sci., Polym. Chem. Ed. 31, 1973 (1993)
(Patent Reference 1)
U.S. Pat. No. 5,403,897
(Patent Reference 2)
Japanese Unexamined Patent Application, First Publication No. 2000-344877
(Patent Reference 3)
Japanese Unexamined Patent Application, First Publication No. 2001-335623
(Patent Reference 4)
U.S. Pat. No. 5,686,540